The reason this is not getting a lot of attention is the same reason any
one GUI project isn't getting a lot of attention, everyone has their own
desire for which toolkit to build from and everyone is busy. On top of
that GtkD exists, while it currently uses an older version of Gtk it is
there and will likely move to Gtk 3.0 when the author has some more time.
The benefits of 'GObject Introspection' are of no consequence since it
already works without them.
I do not speak for the author or his plans, but at this time moving it up
to 3.0 is just more work and time the author doesn't have.

The reason this is not getting a lot of attention is the same reason any
one GUI project isn't getting a lot of attention, everyone has their own
desire for which toolkit to build from and everyone is busy. On top of
that GtkD exists, while it currently uses an older version of Gtk

gtk2 seems to be not an older version, but current version, gtk3 being just a
plan.